This invention relates to control systems in general and more particularly to a control system which provides improved positioning accuracy at high positioning rates.
Control devices which have an integrating positioning drive, e.g., motor and gear train, which has backlash and which exhibits a lag, i.e., a period over which the drive moves after it is deenergized and which includes a feedback input providing a signal proportional to the position of a member being controlled are known. The typical integrating positioning drive includes a positioning motor, preferably an electric motor having a reduction gear which can comprise several stages. The reduction gear transforms the relatively high speed of the positioning motor into the linear or angular motion required for moving a controlled member, for example, a damper or a gate valve, through its positioning range. However, in addition to electric motors pneumatic or hydraulic positioning drives which are controlled by pulse sequences, and are thus actuated in a stepwise manner, can also be considered within the scope of the present invention. In these latter devices, the transmission path can also include backlash.
For control reasons, positioning drives for control purposes should be capable of high positioning rates and of high resolution in positioning.
It is relatively simple to achieve high positioning rates. However, to achieve such and at the same time insure that the control member reaches a predetermined set position accurately necessitates a relatively complex system for braking the positioning drive. Theoretically, a control system should respond even to very small changes of the input variable to the control device. In other words, very small control differences must be converted into a corresponding change of the variable, i.e., the position of the control member. In practice, particularly in large positioning drives, this requirement cannot be realized since positioning motors cannot be built without inertia. As a result the motor cannot convert arbitrarily short positioning pulses into rotary motion and therefore, into positioning travel. Furthermore, because of the large starting currents which electric motors draw, the motor should also not be switched off during the starting phase. That is to say, it should not be switched off from the time it is switched on until it has reached its nominal speed, in order to save the contacts of the switching devices and to avoid excessive heating. For these reasons, the switching pulses supplied at the controller output must not fall below a minimum duration. In turn, the minimum "on" time also determines the smallest possible positioning steps for drives without backlash and therefore, the positioning travel resolution which is possible.
If a positioning motor does not immediately stop after being deenergized, but continues to run down slowly because of its rotational energy, this lag further increases the positioning steps. The magnitude of this lag depends on the size of the motor, the speed, the friction forces of the transmission and the load torque. In order to avoid such uncontrollable lag of the positioning motor, electric motor control drives presently employed in the process industry are frequently equipped with electromechanical or electronic braking devices. However, these devices are relatively expensive and/or are subject to wear.
In the case of electric motor positioning drives using electromechanical brakes, the minimum "on" pulse length therefore must be equal to or greater than the sum of the pickup delay of the power switching device, the time required for the motor to reach operating speed and the lifting time of the brakes. In practice, the minimum pulse length is about 200 ms. In electronic braking devices and solid-state power switches, the minimum "on" time is 50 to 100 ms, depending on the size of the motor.
If, the smallest positioning step should, for instance, not exceed 0.3% of the positioning range, positioning times of more than 15 seconds for the full positioning range would be obtained with the minimum "on" times mentioned. However, such positioning times are too long for dynamically controlling control systems with time constants on the order of 1 s, for instance, flow or pressure control systems.
Thus, the problem, of improving a control device of the type mentioned at the outset, in such a manner that a high positioning rate as well as an extremely high resolution are achieved, arises.